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Send stderr to stdout

Send any error output to the same place as stdout.

COMMAND 2> &1

This is not so useful in itself when just running in the console alone. But more useful when using crontab, tee or writing to a file.

Send stderr and stdout to the same file

COMMAND > stdout_and_sterr.txt 2> &1

The &1 is a pointer to where stdout is currently going.

Shorthand for the above.

COMMAND &> stdout_and_sterr.txt

Apparently supported in all shells.

From askubuntu.com question.

Send stderr and stdout to different files

COMMAND > stdout.txt 2> stderr.txt

Append output

Append stdout to a file.

COMMAND >> stdout_and_sterr.txt

Also append stderr to that file.

COMMAND >> stdout_and_sterr.txt 2> &1

From SO question

Hide all output from a command

COMMAND > /dev/null 2> &1

e.g.

if command -v node /dev/null > 2> &1; then
  echo 'Node is installed!'
else
  echo 'Node is not installed :('
fi